While multiple cardiac progenitor cell populations expressing cKit+ appear to be robust platforms for cardiac repair, considerable controversy exists regarding the role of myocyte regeneration from direct differentiation of cKit+ cells vs. their ability to stimulate endogenous myocyte proliferation. In rodent infarcts, cKit+/Lin- cardiac stem cells (CSCs) isolated from atrial biopsies differentiate into new myocytes with myocardium arising from the transplanted CSCs. In contrast, we and others demonstrated that cKit+ cardiosphere derived cells (CDCs) isolated from ventricular biopsies facilitate repair indirectly b stimulating endogenous myocyte proliferation. Small clinical trials demonstrate favorable effects of both approaches in reducing scar volume and increasing myocardial mass yet differ in that CSCs improve ejection fraction whereas CDCs do not. A major gap in our understanding exists due to the paucity of preclinical studies performed in large animals addressing fundamental questions related to the number and sources of new myocytes. Completed work by our laboratory has demonstrated the feasibility of accomplishing this using allogeneic CDC transplants in sex mismatched donors. Accordingly, we will use allogeneic cells with immunosuppression in a blinded trial design to address the central hypothesis that uncommitted cKit+/Lin- atrial CSCs elicit cardiac regeneration by directly producing new myocytes while cKit+ CDCs committed to a cardiac lineage elicit cardiac repair by stimulating endogenous cardiomyocyte proliferation. Using a translational large animal model of a healed infarction in miniswine with chronic LV dysfunction we will employ global intracoronary stem cell infusion without transiently occluding the coronary artery and evaluate the impact of infusing cells to dysfunctional remote regions as well as the infarct to address three Specific Aims: Aim 1 will determine whether allogeneic atrial cKit+ CSCs administered with cyclosporine can improve myocardial function to the same extent as autologous CSCs and com- pare their effectiveness to allogeneic ventricular cKit+ CDCs; Aim 2 will quantify the contribution of myocytes derived from CSCs and CDCs to myocyte regeneration using a lentiviral triple reporter gene construct (PET, fluorescence, bioluminescence), sex mismatched donor/recipients, immunohistochemistry and morphometry ; Aim 3 will evaluate whether endogenous myocyte proliferation is the major mechanism in cKit+ stem cell repair by determining whether cyclosporine immunosuppression can be discontinued after eliciting functional improvement with allogeneic cKit+ CSCs and CDCs. The studies will provide quantitative insight comparing the ability of two competing cKit+ stem cell platforms to regenerate new myocytes in an animal model with a heart size and characteristics similar to patients. The project has high translational significance since demonstrating the feasibility to use cKit+ CDCs or CSCs from an allogeneic donor would provide an off-the-shelf approach for cardiac stem cell therapy. This could be implemented using standard cardiac catheterization approaches and make cardiac cell therapy widely available to treat patients with ischemic heart disease.